The chitin synthase regulator CSR-3 and the SO protein function together in cell‑cell fusion and stress-induced cell wall remodeling in Neurospora crassa

Chitin is an important component of the fungal cell wall, whose synthesis is mediated by chitin synthases and chitin synthase regulators with defined tasks. For instance, response to cell wall stress usually requires chitin synthesis which is controlled by the cell wall integrity (CWI) pathway. In the ascomycete Neurospora crassa the MAP kinase MAK-1 and the potential scaffold protein SO are part of CWI signaling. These proteins also play an essential role in cell-cell fusion of germinated spores. This process results in an interconnected, supracelluar network, the mycelium. During fusion the cell walls of the interacting partners are partially degraded before the opposing plasma membranes merge. This cell wall remodeling bears the risk of cell lysis and probably requires a fine-tuned equilibrium between chitin synthesis and depletion.

A Y2H-screen revealed that the SO protein physically interacts with the two upstream MAP kinases of three-tiered MAK-1 MAP kinase module and the chitin synthase regulator 3 (CSR-3). When fusing germlings achieve physical contact, CSR-3 and SO together with MAK-1 and it upstream kinase MEK-1 co-localize at the fusion point until cell merger has been completed. A csr-3 knockout mutant tends to lyse during this process, suggesting a supporting role of CSR-3 during fusion pore formation. In addition, exposing cells to cell wall stress results in recruitment of those factors to the cell surface. CSR-3 and its potential target chitin synthase 2 also seem to play a role in septation in germlings and in hyphae.

Based on these observations, we hypothesize that shared molecular factors are involved in cell-cell fusion and stress induced cell wall remodeling.